Stabilized polyester compositions

ABSTRACT

Saturated polyester compositions stabilized against thermal degradation by the addition of small amounts of a synergistic stabilizing combination of a 5 or 6 membered, cyclic organic carbonate and an antioxidant selected from the group consisting of certain alkylated phenols, polyphenols, alkylphosphites, arylphosphites, aromatic amines and aromatic polyamines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application,Ser. No. 348,217, filed Apr. 5, 1973 now U.S. Pat. No. 3,888,817 andassigned to the assignee of the instant application.

BACKGROUND OF THE INVENTION

This invention relates to novel, heat-stable polyester compositions.More particularly, this invention relates to saturated polyestersstabilized against thermal and oxidative and hydrolytic degradation byuse of a synergistic combination comprising a cyclic, organic carbonateand an antioxidant.

The term "saturated polyester" encompasses a wide variety of materialswhich are of significant industrial and economic importance. Well-knownpolyester materials include polyester fibers, polyester tire cord,plasticizers, alkyds and polyester adhesives. For the purpose of thisinvention it is intended to include those polyesters having linearchain-like structures derived by reacting compounds such as dicarboxylicacids or their acid functioning derivatives with dihydric alcohols or byheating omega-hydroxycarboxylic acids; and those polyesters havingcross-linked, three dimensional structures derived by reacting di- orpolycarboxylic acids with diols or polyols; said polyesters orcopolyesters may contain alkyl or aryl or cycloalkyl or other groupswhich are not ethylenically unsaturated.

It is well-known to those skilled in the art that most polyesters arerelatively stable materials as compared with other types of polymers,e.g. polyvinyl acetate, polystyrene, polyurethanes, etc. It is also wellknown, however, that when polyesters are exposed to high temperatures,high humidity and air, as they often must during processing andapplication from the molten state, they tend to degrade badly withdetrimental loss of physical and chemical properties. This conditionoften results in inferior products at best, or worse, it renders thesematerials useless in the intended application thereby contributing tosubstantial economic losses to manufacturer and end-user alike.

The various degradation processes have been widely studied, and a numberof stabilizers, e.g. acidity reducing agents, antioxidants, etc., havebeen suggested to minimize degradation; all of these stabilizers,however, have a limited stabilizing effect at best and are relativelyexpensive.

SUMMARY OF THE INVENTION

It is, thus, the primary object of this invention to provide saturatedpolyester compositions having unusually outstanding stabilitycharacteristics. Another object of the present invention is to providestabilized polyester compositions useful as fibers, film, tire cord,polyurethane intermediates and adhesives. Still, another object is toprovide stabilized polyester compositions having improved properties,e.g. color, viscosity stability and resistance to environmental attack.Various other objects and advantages of this invention will becomeapparent from the following description thereof.

It has now been found that polyesters having all of the above describedproperties can be obtained by adding to the polyester or copolyester aproportionately small amount of a synergistic combination comprising atleast one 5 or 6 membered cyclic organic carbonate and at least oneantioxidant selected from the group consisting of alkylated phenols,alkylidene bisphenols, alkylphosphites, arylphosphites, aromatic amines,and aromatic polyamines.

In general, the carbonate components of the stabilizing combinations ofthis invention include those compounds having a 5 or 6 membered ringwhich contains the characteristic carbonate, ##STR1## group. Among suchcompounds are included the alkylene carbonates having saturated rings,e.g., ethylene carbonate, propylene carbonate, the substituted andunsubstituted alkylene carbonates, etc.; and the vinylene carbonateshaving unsaturated --C=C-- linkages within the ring, e.g., vinylenecarbonate, propylvinylene carbonate, ethylvinylene carbonate, etc. Thecarbonates of the types having 5 membered rings as discussed above maybe typically represented by the following general structures: ##STR2##or ##STR3## wherein the R's may be H; an aromatic group; an aliphaticgroup which may be saturated or unsaturated; or alicyclic group, saidgroups having from 2 to 18 carbon atoms. R₁, R₂, R₃, and R₄ may be thesame or different.

The useful carbonates having 6 membered rings may be typicallyrepresented by the following general structure: ##STR4## wherein the R'sare as defined above.

In a varied aspect in the 5 and 6 membered ring structures the various Rgroups may be interconnected to give cyclic carbonates having openstructures, condensed ring structures, polycyclic structures, etc. Itwill be appreciated by those skilled in the art that compoundscontaining more than one cyclic carbonate group will also be effectivestabilizers when used in conjunction with the hereinbelow describedantioxidants. Examples of such compounds include diglycerol carbonate,polyvinylene carbonate, etc. Also compounds containing both cycliccarbonate and antioxidant groups, e.g., pyrogallol carbonate(1-hydroxy-2,3-phenylene carbonate), tris(2,3-carbonyldioxyphenyl)phosphite (the reaction product of pyrogallol carbonate and phosphorustrichloride), etc. The preparation and identification of these cycliccarbonates and others are described in the literature, e.g. Methoden DerOrganischen Chemie, Band VIII, pp. 107-110. In general, any compoundcontaining 5 or 6 member cyclic carbonate or carbonates may be used,provided that other groups present in the same compound do not adverselyeffect the stabilities of the carbonate moiety nor the saturatedpolyester.

In general, the antioxidant components of the stabilizing combinationsherein are those compounds which are capable of inhibiting oxidation.

Such compounds, which are either primary antioxidants, or secondaryantioxidants which destroy peroxides and also regenerate primaryantioxidants include alkyl substituted phenols, bisphenols, substitutedbisphenols, thiobisphenols, polyphenols, thiobisalkylates, aromaticamines, and organic phosphites and polyphosphites, as describedhereinbelow.

One group of alkyl substituted phenols may be structurally representedas follows: ##STR5## wherein R is H, an alkyl group, or an alicyclicgroup, and R₁ may be alkyl or a symmetrical or unsymmetrical ester grouplinked to the phenolic ring by a terminal carbon; said groups containingfrom 1-18 carbon atoms. Examples of such compounds are 2,6-di-tertiary-butyl-p-cresol and octadecyl beta (4-hydroxy-3, 5-ditert-butylphenyl) propionate, etc.

Among the useful substituted bisphenols and thiobisphenols may be thosestructurally represented as follows: ##STR6## wherein Z is a methylenegroup, an alkylidene group containing from 2-18 carbon atoms, orsulphur. R is as defined in structure III, supra. Examples of suchcompounds are 2, 2' - thiobis-(6-t-butyl-3-methylphenol) and 4, 4'-thiobis (6-t-butyl-3-methylphenol), etc.

One type of the useful polyphenols may be structurally represented asfollows: ##STR7## wherein R₁ is defined in structure III, supra.Examples of such compounds are pentaerythrityltetrakis[beta-(4-hydroxy-3, 5-di-tert-butylphenyl)] proprionate and thelike.

In another aspect, the useful polyphenols may be of the type having themore complex structure represented as follows: ##SPC1##

wherein R is as defined above in structure III and R₂ is a tertiarybutyl group.

Among the useful dialkyl thiodiesters are included those which may bestructurally represented as follows: ##STR8## wherein R₄ is an alkylgroup having from 4 to 20 carbon atoms.

Among the useful organic phosphites and phosphites are includeddiisooctyl phenyl phosphite, trioctyl phosphite, triisooctyl phosphite,tricyclohexyl phosphite, triphenyl phosphite, distearyl pentaerythrityldiphosphite, bis-neopentyl glycol triethylene glycol diphosphite, etc.

Among the useful aromatic amines are included the primary polyaminessuch as p,p'-diaminodiphenylmethane, benzidine, p-phenylenediamine,m-phenylenediamine, m-toluenediamine; the diarylamines such asN-phenyl-alpha-naphthylamine, N-phenyl-beta-naphthylamine,p-isopropoxydiphenylamine; the bisdiarylamines such asN,N'diphenyl-p-phenylenediamine,N,N'-di-beta-naphthyl-p-phenylenediamine; mixtures containingdiarylamines such as mixture of N-phenyl-alpha-naphthylamine and2,4-diaminotoluene, a mixture of N,N'-diphenylethylenediamine andN,N'-diphenyl-p-phenylenediamine; modified bisdiarylamines such asN-cyclohexyl-N'-phenyl-p-phenylenediamine; alkylated diarylamines suchas monooctyl- and dioctyldiphenylamine mixtures; mixtures containingalkylated diarylamines such as mixtures of octyldiphenylamines and wax;ketone-diarylamine condensation products such as diphenylamine-acetonecondensation products; dihydroquinolines such as1,2-dihydro-2,2,4-trimethyl-6-phenylquinoline; mixtures ofdihydroquinolines and diarylamines such as mixtures of polymerized1,2-dihydro-2,2,4-trimethylquinoline andN,N'-diphenyl-p-phenylenediamine; aldehyde-amine condensation productssuch as aldol-alpha-naphthylamine condensation products,butyraldehyde-aniline condensation products; alkylarylamines such asN,N'-diphenylethylenediamine and aldehyde imines such asN,N'-disalicylalethylenediamine, etc.

As previously indicated, any saturated polyester, or copolyester may bestabilized with a suitable cyclic organic carbonate and antioxidantcombination. Such stabilizable materials include the cross-linked threedimensional esters obtained by reacting polycarboxylic acids withpolyols or those prepared from bifunctional reactants only, i.e., whenomega-hydroxy carboxylic acids are heated or when a dibasic acid reactswith a dihydric alcohol to produce a linear polyester. Among suchstabilizable polyesters may be included those prepared by the generalmelt polymerization techniques described in Whinfield et. al., U.S. Pat.No. 2,465,319 and inter alia, Snyder, U.S. Pat. No. 2,623,031. Otherpolyesters suitable for use in the practice of this invention are thoseprepared by reacting two moles of the half ester derived from amono-basic acid and a glycol with one mole of a dibasic acid asdisclosed in U.S. Pat. No. 2,575,196.

Additional information relating to the preparation of saturatedpolyesters, including the vulcanizable-elastomer type, are described byB. Bolding in "Polymers and Resins," D. Van Nostrand Co., Inc. 1959,283- 9.

The stabilizing combinations of this invention can be intimately admixedwith the molten polyester in an inert atmosphere. That is to say, thecarbonate and the antioxidant components may be incorporated,individually or in combination, with the molten polyester just prior todischarge from the reactor, as preferred in manufacturing operations, orwith the remelted polyester. Said components may be added as theirisolated compounds or, in order to achieve more rapid dispersion, theycan be added in the form of a solution. The actual temperature and timerequired to achieve adequate dispersion will depend greatly on thecomposition of the particular polyester and its viscosity. Nevertheless,it is necessary, in all instances, that the temperature be maintainedabove the melting point of the polyester and stirring be continued longenough to ensure a smooth, homogeneous product.

As an alternate method it may be preferred to admix the crystallinecarbonate component and the antioxidant component with the dry polyesterresin and then store the mixture until further processing, i.e.,extrusion, injection molding, etc., whereupon stabilization occurs.

With regard to proportions, the total carbonate and antioxidantstabilizing combination may be added in an amount ranging from 0.1 to5.0 per cent, by weight, of the synergistic-stabilizing combination.Within said combination, the carbonate may comprise between 90-10 percent of the total weight. The actual amount of each component used isleft up to the discretion of the practitioner. Normally, in order toprovide substantially improved stability to the various polyesters, itis sufficient to employ a single carbonate with a single antioxidant atequal concentrations. But, for the purposes of this invention, it ispreferred that the amount of the carbonate component, i.e., one or morecarbonates, comprise between 50 and 80 per cent, by weight, of theentire synergistic-stabializing combination.

The effectiveness of the novel, synergistic, stabilizing combinations ofthis invention was determined by measuring the decrease in intrinsicviscosity (I.V.) of the polyester before and after heating in an opencontainer in an oven for a fixed period. I.V. measurements (expressed indeciliters per gram (dl/gm) were made using a Cannon-Fenske capillaryviscometer (size 100) as described by P. J. Flory in "Principles ofPolymer Chemistry," VII, 1953, 309-310. The solvent used was 1, 1, 2,2-tetrachloroethane, and the viscosity of the polyester at each phase ofthe test was measured at concentrations of 0.5, 1.0 and 1.5 grams per100 ml. of said solvent. Any apparent decrease in molecular weight, dueto thermal degradation, is indicated by a decrease in I.V.

The polyesters stabilized with the novel, synergistic stabilizingcombinations of this invention exhibit lasting resistance to oxidativeor hydrolytic or thermal degradation.

The novel, thermally resistant polyesters of this invention mayeffectively be utilized in a variety of commercial uses, i.e. syntheticfibers, films, tire cords, etc. These polyesters are most readily usefulwherein thermal and oxidative degradation has heretofore been a problem.Such uses include ironable clothing articles, and, in particular,hot-melt adhesive compositions.

The polyesters may be further treated or otherwise processed withoutincurring any deleterious effects on their ability to resist thermaldegradation. For example, their thermal stability is not impaired whenoptional ingredients such as fillers, plasticizers, colorents, etc., arecalled for. Similarly, no adverse effects are realized when thepolyester is subjected to injection molding or biaxial orientation,e.g., stretching in two directions perpendicular to each other to form afilm.

The advantage afforded by the stabilizers of the present invention aremany: For example: 1. polyesters and copolyesters having superior heatstability can now be made and used in areas, e.g. hot melt adhesives,where degradation, due to prolonged heating in the melt, has limitedtheir use. 2. physical and chemical properties of the stabilizedpolyesters are essentially maintained even after prolonged heating inair. 3. skin formation (surface cross-linking) is virtually eliminated.4. there is very little change in melt viscosity. 6. cost advantagesresult due to the generally lower cost of the cyclic carbonates ascompared with the cost of most antioxidants. These and other advantageswill become apparent from the following examples, which furtherillustrate but do not limit the scope of the present invention.

In these examples, all quantities are given as parts, by weight, unlessotherwise specified.

EXAMPLE I

This example illustrates the preparation of a thermally stablecopolyester, using an alkylene carbonate and thiobisphenol antioxidantcombination in accordance with this invention.

A bulk quantity of a copolyester was prepared by reacting two aryldicarboxylic acids, an anhydride of an aryl dicarboxylic acid, and analkanediol as set forth below. In this instance, a proportionately smallamount of antioxidant was used.

Into a reaction vessel equipped with a mechanical stirrer, thermometer,nitrogen inlet, 18 inch Vigreux column, distillation head withthermometer, a Dean and Stark 20 ml. distillation receiver and acondenser, there were introduced the following ingredients:

    ______________________________________                                        Ingredient                Amount                                              ______________________________________                                        Dimethyl terephthalate   970.0   g.                                           1,4-Butanediol           1440.0  g.                                           Zinc acetate (dihydrate) 1.3     g.                                           Antimony trioxide        1.3     g.                                           4,4'-Thiobis(6-tert-butyl-m-cresol)                                                                    0.65    g.                                           Toluene                  50.0    ml                                           ______________________________________                                    

Under continuous agitation, the above ingredients were heated to andmaintained at 219° C. for 3.5 hours to assure completetransesterification. Then residual methanol, toluene, and a smallquantity of tetrohydrofuran were distilled off and discarded. Thereafterthe contents of the reaction vessel were cooled to 140° C., and therewere added thereto 664.0 grams of isophthalic acid, 148.0 grams ofphthalic anhydride, and 50.0 ml. of xylene. Agitation was resumed, andthe mixture was heated to and maintained at 240° C. to remove the waterduring refluxing of the xylene. The reaction was continued until an acidnumber of 8.4 (mgs of KOH/gm of sample) was indicated. Then the pressurewas gradually reduced to between 1.0 and 2.0 mm. mercury. Thetemperature was gradually increased to and maintained at 250°-255° C.for 2.5 hours. 20 grams of chips obtained from the copolyester describedabove were placed in a 100 ml. beaker. The beaker was then mounted in anoil bath and equipped with a mechanical stirrer and a nitrogen supplysource. Said beaker was then heated to about 200° C. to remelt thepolyester. Then in a nitrogen atmosphere, there was added to saidbeaker, based on the weight of the polyester, 0.6 per cent of ethylenecarbonate and 0.3 per cent of 4,4' thiobis (6-tert-butyl-m-cresol). Themolten mixture was stirred for about 4 minutes and then tested forthermal stability. Initial intrinsic viscosity (I.V.) was determinedaccording to the method described hereinabove. The beaker was thenstored in an oven set at 400° F. for 72 hours. Following the holdingperiod, a final intrinsic viscosity measurement was made.

To prepare controls, three additional 20 gram portions of the polyesterdescribed above were placed in three separate beakers designated A, B,and C. These beakers were each mounted in an oil bath, equipped with amechanical stirrer, and provided with a nitrogen supply source. Saidbeakers were then heated to about 200° C. to remelt the polyester. Themolten polyester contained in beaker A remained unmodified to serve as ablank control. To beaker B there was added 0.9 per cent, by weight ofthe polyester, of the ethylene carbonate only. And to beaker C there wasadded 0.9 per cent, by weight of the polyester of the 4,4' thiobis(6-tert-butyl-m-cresol) only. Then intrinsic viscosity measurements ofthe three controls were made before and after the 72 hour holding periodat 400° F. as described above. Test results of the controls comparedwith those of the sample prepared above are as set forth in Table Ibelow.

                  TABLE I                                                         ______________________________________                                                   Intrinsic Viscosity                                                           (deciliters/gram)                                                  Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Test Sample  0.398     0.338     -15.1                                        Control A    0.398     0.240     -38.7                                        Control B    0.398     0.272     -31.7                                        Control C    0.398     0.302     -24.1                                        ______________________________________                                    

As indicated above, the test sample of the polyester modified with thesynergistic stabilizing combination incurred the least percentagereduction in intrinsic viscosity and was therefore most resistant tothermal degradation.

EXAMPLE II

The procedure of Example I was repeated using ethylene carbonate andp-phenylene diamine antioxidant combination in accordance with theinvention. Controls A, B and C were also prepared as in Example I. Themolten polyester contained in beaker A remained unmodified to serve as ablank control. To beaker B there was added 0.9 percent, by weight of thepolyester, of the ethylene carbonate only. To beaker C there was added0.9 percent, by weight of the polyester, of the p-phenylene diamineonly. Viscosity measurements of the test sample and of the threecontrols were made before and after 72 hour holding periods at 400° F.The results are shown in Table II below.

                  TABLE II                                                        ______________________________________                                                   Intrinsic Viscosity                                                           (deciliters/gram)                                                  Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Test Sample  0.398     0.356     -10.5                                        Control A    0.398     0.252     -39.2                                        Control B    0.398     0.268     -32.7                                        Control C    0.398     0.285     -28.4                                        ______________________________________                                    

The results clearly show that the test sample of the polyester modifiedwith the synergistic stabilizing combination incurred the leastpercentage reduction in intrinsic viscosity and was therefore mostresistant to thermal degradation.

EXAMPLES III-VI

These examples illustrate the usefulness of an alkylene carbonate incombination with varied antioxidants as stabilizing combinations in asaturated copolyester.

A series of 4 sample copolyesters and a suitable control for each samplewere prepared as follows:

Into each of eight 100 ml. beakers there were introduced 20 grams of thecopolyester prepared in Example I. The beakers were then separated intopairs, one of each pair being designated the test sample and the other,the respective control. In each case the particular alkylene carbonateand antioxidant combination was added to the sample polyester, and asimilar alkylene carbonate or the antioxidant was added to the controlpolyester in the manner employed in Part A of Example I. Compositions ofthe various samples were as follows:

    ______________________________________                                                       Sample No. and Amount                                                         (% by weight of polyester                                      Modifying Agent  III     IV      V     VI                                     ______________________________________                                        Ethylene Carbonate                                                                             0.6     0.6     0.6   0.6                                    Triphenyl phosphite                                                                            0.3     --      --    --                                     Pentaerythrityl tetrakis-                                                     [beta- (4-hydroxy-3,5-di-                                                     tert-butylphenyl)]propionate                                                                   --      0.3     --    --                                     1,3,5-trimethylene, 2,4,6-                                                    tris (3,5- tert-butyl-4-                                                      hydroxybenzyl) benzene                                                                         --      --      0.3   --                                     N-phenyl-alpha naphthylamine                                                                   --      --      --    0.3                                    ______________________________________                                    

The above described copolyesters, a standard control comprised of theunmodified copolyester, and a specific control for each sample, weretested for thermal stability according to the method described inExample I. Said specific control, in each case, contained 20 grams ofthe copolyester base modified with 0.9 per cent, by weight, of thecopolyester, of the same antioxidant employeed in the correspondingsample. The latter copolyesters were designated control (III), control(IV), control (V), and control (VI), respectively. Test results are setforth in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                   Intrinsic Viscosity                                                           deciliters/gram)                                                   Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Control (Stnd.)                                                                            0.398     0.240     -38.7                                        Sample III   0.398     0.340     -14.5                                        Control (III)                                                                              0.398     0.309     -22.4                                        Sample IV    0.398     0.309     -22.4                                        Control (IV) 0.398     0.298     -25.1                                        Sample V     0.398     0.312     -21.6                                        Control (V)  0.398     0.293     -28.2                                        Sample VI    0.398     0.350     -12.0                                        Control (VI) 0.398     0.313     -21.3                                        ______________________________________                                    

As indicated by the data summarized above, the samples containing thestabilizing combinations showed the least degradation.

EXAMPLE VII-X

These examples illustrate the effectiveness of additional carbonates,when used with an arylphosphite antioxidant, to stabilize copolyestersaccording to this invention.

A sufficient quantity of the copolyester base prepared in Example I wasused to prepare four test samples and six controls. Each test samplecomprised 20 grams of the copolyester having incorporated therein theparticular synergistic stabilizing combination. Each control, exceptthat having 20 grams of the unmodified copolyester only, comprised 20grams of the copolyester having incorporated therein either a singlecarbonate or the antioxidant component only.

To make the test samples and the controls having the single componentstherein, the procedural steps of Example I were repeated, except hereinthe 4,4'thiobis(6-tert-butyl-m-cresol) and ethylene carbonate werereplaced with triphenyl phosphite and varied carbonate combinations asfollows:

    ______________________________________                                                       Sample No. and Amount of Agent                                                (%, by weight, of the copolyester)                             Modifying Agent  VII     VIII    IX    X                                      ______________________________________                                        Triphenyl phosphite                                                                            0.6     0.6     0.6   0.6                                    Trimethylene carbonate                                                                         0.6     --      --    --                                     Neopentylene carbonate                                                                         --      0.6     --    --                                     (2,2-dimethyl trimethylene                                                    carbonate)                                                                    Catechol carbonate                                                                             --      --      0.6   --                                     (o-phenylene carbonate)                                                       Styrene carbonate                                                                              --      --      --    0.6                                    ______________________________________                                    

The above described copolyesters, a standard control comprised of theunmodified copolyester, and a specific control for each sample, weretested for thermal stability according to the method described inExample I. Herein said specific control, in each case, contained 20grams of the copolyester base modified with 1.2 per cent, by weight, ofthe copolyester of the particular carbonate employed in thecorresponding sample. The latter copolyesters were designated Control(VII), Control (VIII), Control (IX), and Control (X) respectively. Thethermal stability of the test samples and the controls were determinedand evaluated in the manner set forth in Example I. Results arepresented in Table 3 below.

                  TABLE 3                                                         ______________________________________                                                   Intrinsic Viscosity                                                           (deciliter/gram)                                                   Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Control (Stnd)                                                                             0.398     0.268     -32.7                                        Control (phosphite                                                             only)       0.398     0.324     -18.6                                        Sample VII   0.398     0.369     - 7.3                                        Control (VII)                                                                              0.398     0.287     -27.9                                        Sample VIII  0.398     0.364     - 8.5                                        Control (VIII)                                                                             0.398     0.277     -30.4                                        Sample IX    0.398     0.375     -15.1                                        Control (IX) 0.398     0.329     -17.3                                        Sample X     0.398     0.338     -15.1                                        Control (X)  0.398     0.292     -26.7                                        ______________________________________                                    

The data summarized above illustrates that the samples containing thestabilizing combinations incurred less degradation than the respectivecontrols.

EXAMPLE XI

This example illustrates the usefulness of a synergistic combinationcomprising an alkylene carbonate and two antioxidants as a copolyesterstabilizer.

Using additional 20 gram portions of the copolyester prepared in ExampleI and the procedure described in Part A of that same example, a testsample having 0.6 per cent, by weight, of ethylene carbonate incombination with 0.15 per cent, by weight, of pentaerythrityltetrakis[beta-(4-hydroxy-3,5-tert butyl phenyl)] propionate and 0.15 percent, by weight, of dilauryl thiodipropionate was prepared. A controlhaving the same combination of antioxidants but no carbonate was alsoprepared. The resulting products were then tested for thermal stabilityin the manner set forth in Example I. The test results compared asfollows in Table 4.

                  TABLE 4                                                         ______________________________________                                                   Intrinsic Viscosity                                                           (deciliters/gram)                                                  Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Test Sample  0.398     0.325     -18.3                                        Control      0.398     0.290     -27.2                                        ______________________________________                                    

Based on the above data, it is thus seen that the resistance to thermaldegradation of a copolyester may also be improved, when two antioxidantsare employed with the carbonate to make-up the synergistic stabilizingcombination.

EXAMPLE XII

This example illustrates synergistic stabilizing combinations havinggreater amounts of the carbonate component.

Using additional 20 gram portions of the polyester prepared in Example Iand the test procedure set forth in that same example, a sample having1.0 per cent, by weight, of ethylene carbonate and 0.3 per cent, byweight, of 4,4' -thiobis (6-tert-butyl-m-cresol); and a control havingthe same amount of the latter ingredient only, were prepared. Thethermal stability of the test sample and the control were thendetermined according to the method employed in Example I. Test resultsare tabulated in Table 5, show that the test samples had improvedthermal stability.

                  TABLE 5                                                         ______________________________________                                                   Intrinsic Viscosity                                                           (deciliters/gram)                                                  Material Tested                                                                            Initial   Final     % Change                                     ______________________________________                                        Test Sample  0.421     0.380     - 9.7                                        Control      0.421     0.327     -22.3                                        ______________________________________                                    

Summarizing, it is thus seen that this invention provides a novelpolyester composition characterized by its unusually greater resistanceto thermal and environmental degradation, due to the presence of acarbonate ad antioxidant stabilizing combination. The resultingcombination exhibits outstanding synergistic stability performance,i.e., the performance of the combination substantially exceeds the sumtotal of the performances exhibited by the individual componentsthereof, when used separately. It will be understood that variations inthe proportions of the carbonate and the antioxidant components, as wellas the particular stabilizable polyester to which the combination isadaptable, may be made without departing from the spirit of theinvention defined in the following claims.

What is claimed is:
 1. A saturated polyester composition stabilizedagainst thermal degradation comprising:a. a polyester blended with b.0.3 to 5.0 per cent, by weight of the polyester, of a combinationcomprising1. 10 to 90 percent, by weight, of at least one cyclic organiccarbonate having at least one cyclic organic carbonate group permolecule containing 5 - 6 atoms in the ring, and
 2. 2. the remainder ofsaid combination comprising at least one aromatic amine antioxidantselected from the group consisting of primary polyamines, diarylamines,bisdiarylamines, mixtures containing diarylamines, modifiedbisdiarylamines, alkylated diarylamines, mixtures containing alkylateddiarylamines, ketone-diarylamine condensation products,dihydroquinolines, mixtures of dihydroquinolines and diarylamines,aldehyde-amine condensation products, alkylarylamines and aldehydeimines.
 2. The stabilized polyester composition of claim 1 wherein saidcarbonate is ethylene carbonate and said antioxidant is p-phenylenediamine.
 3. The stabilized polyester composition of claim 1 wherein saidcarbonate is ethylene carbonate and said antioxidant isN-phenyl-alpha-naphthylamine.